Gas-permeable membrane

ABSTRACT

Gas-permeable membranes comprising a microporous film and a coating on the microporous film, the coating being obtained by coating the microporous film with a liquid coating composition comprising a polymer and hollow polymeric particles dispersed in the composition, and then solidifying the coating. The gas-permeable membrane has a reduced ratio of carbon dioxide permeability to oxygen permeability.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/742,548filed Dec. 19, 2003, which claims the benefit under 35 USC 119(e)(1) ofthe U.S. provisional patent application No. 60/435,567, filed by RaymondClarke and Charles B. Derringer on 20 Dec. 2002. The entire disclosureof each of those applications is incorporated herein by reference forall purposes.

BACKGROUND

The invention relates to gas-permeable membranes suitable for use in thepackaging of respiring biological materials.

Respiring biological materials, e.g. fruits and vegetables, consumeoxygen (O₂) and produce carbon dioxide (CO₂) at rates which depend uponthe stage of their development, the atmosphere surrounding them and thetemperature. In modified atmosphere packaging (MAP), the objective is toproduce a desired atmosphere around respiring materials by placing themin a sealed container whose permeability to O₂ and CO₂ is correlatedwith (i) the partial pressures of O₂ and CO₂ in the air outside thepackage, and (ii) the temperature, to produce a desired atmospherewithin the container. In many cases, the container includes anatmosphere control member (ACM) having a high O₂ transmission rate (OTR)and CO₂ transmission rate (COTR). In controlled atmosphere packaging(CAP), the objective is to produce a desired atmosphere around respiringmaterials by displacing some or all of the air within a container by oneor more gases, e.g. nitrogen, O₂, CO₂ and ethylene, in desiredproportions. For further details of MAP and CAP, reference may be made,for example, to U.S. Pat. Nos. 3,360,380 (Bedrosian), 3,450,542(Badran), 3,450,544 (Badran et al.), 3,798,333 (Cummin et al), 3,924,010(Erb), 4,003,728 (Rath), 4,734,324 (Hill), 4,779,524 (Wade), 4,830,863(Jones), 4,842,875 (Anderson), 4,879,078 (Antoon), 4,910,032 (Antoon),4,923,703 (Antoon), 4,987,745 (Harris), 5,041,290 (Wallace et al.)5,045,331 (Antoon), 5,063,753 (Woodruff), 5,160,768 (Antoon), 5,254,354(Stewart), 5,333,394 (Herdeman), 5,433,335 (Raudalus et al.), 5,460,841(Herdeman), 5,556,658 (Raudalus et al.), 5,658,607 (Herdeman), 5,807,630(Christie et al.), 6,013,293 (De Moor), 6,376,032 (Clarke et al.),6,548,132 (Clarke et al.), and 6,579,607 (Gozukara et al.), copendingcommonly assigned U.S. patent application Ser. Nos. 09/858,190(Publication Number US2002/0090425) and 09/989,682 (Publication NumberUS2002/0127305), Publication Number US2003/0099832, published 29 May2003, International Publication Nos. WO 94/12040 (Fresh Western), WO96/38495 (Landec), WO 99/33658 (Gozukara et al.), WO 00/04787 (Landec)and WO 01/92118 (Landec), and European Patent Applications Nos.0,351,115 and 0,351,116 (Courtaulds). The disclosure of each of thosepatents, applications and publications is incorporated herein byreference for all purposes.

The preferred packaging atmosphere for a respiring material oftendepends on the material and the changes (if any) in the material whichare desired. In some cases, it is desirable for the packaging atmosphereto have a relatively high CO₂ content and a relatively low O₂ content.In order to obtain such a packaging atmosphere in a modified atmospherepackage, it is desirable to make use of an ACM which has a relativelylow COTR/OTR ratio (often referred to herein as the R ratio).

U.S. Pat. No. 5,807,630 (Christie et al.), U.S. Pat. No. 6,579,607(Gozukara et al.) and Publication Number US 2003/0099832 (Borchardt),published May 29, 2003, disclose self-supporting films of controlledpermeability which comprise a film-forming polymer and a porous filler.The filler has a particle size greater than the intrinsic film thicknessof the film-forming polymer, and is present in amount sufficient toreduce the R ratio of the film.

SUMMARY OF THE INVENTION

We have discovered that novel and useful gas-permeable membranes,suitable for use as ACM's in packaging respiring materials, can beobtained by coating a microporous polymeric film with a liquid coatingcomposition comprising

(a) a polymer, and

(b) hollow polymeric particles dispersed in the composition.

The presence of the hollow polymeric particles in the liquid coatingcomposition results in a membrane having a reduced R ratio.

In a first aspect, this invention provides a method of preparing agas-permeable membrane which comprises a microporous film and a solidcoating on the microporous film, the method comprising

-   -   (A) forming a liquid coating on the microporous film, the liquid        coating being composed of liquid coating composition which        comprises        -   (a) a first polymer, and        -   (b) hollow particles which (i) are dispersed in the coating            composition, and (ii) are composed of a polymeric            composition comprising a second polymer, the second polymer            being different from the first polymer; and    -   (B) solidifying the liquid coating on the microporous film.

In a second aspect, this invention provides a gas-permeable membranewhich comprises

-   -   (1) a microporous film, and    -   (2) a solid coating on the microporous film, the coating        comprising        -   (a) a matrix comprising a first polymer, and        -   (b) hollow particles which (i)) are composed of a polymeric            composition comprising a second polymer, (ii) are dispersed            in the matrix, and (iii) have a maximum dimension which is            at most 50% of the thickness of the solid coating, the            second polymer being different from the first polymer.

In a third aspect, this invention provides a gas-permeable membranewhich comprises

-   -   (1) a microporous film, and    -   (2) a solid coating on the microporous film, the coating        comprising        -   (a) a matrix comprising a first polymer, and        -   (b) a plurality of microscopic voids which            -   (i) provide continuous pathways for the transmission of                oxygen and carbon dioxide through the coating, and            -   (ii) are at least partly defined by walls composed of                the second polymer.

The gas-permeable membranes of the second and third aspects of theinvention can be prepared by the method of the first aspect of theinvention. The membranes of the third aspect of the invention areobtained when the solidification step (B) involves heating which atleast partially melts at least some of the hollow polymeric particles sothat they fuse together to form a plurality of microscopic voids. Thus,it is possible for the solid coating of the membranes of the second andthird aspect of the invention to include both (i) hollow polymericparticles which are the same as or similar to the hollow polymericparticles in the coating composition and (ii) microscopic voids formedby fusion of hollow polymeric particles.

In a fourth aspect, this invention provides a container which can besealed around a respiring biological material and which includes one ormore ACM's, at least one of the ACM's comprising a gas-permeablemembrane prepared by the method of the first aspect of the inventionand/or as defined in the second and/or third aspect of the invention.Generally, the container is such that, after the container has beensealed around the biological material, at least 50%, often at least 75%,of the oxygen which enters the interior of the sealed package passesthrough the one or more ACM's.

In a fourth aspect, this invention provides a package which comprises

-   -   (a) a sealed container, and    -   (b) within the sealed container, a respiring biological material        and a packaging atmosphere around the biological material;        the sealed container including one or more ACM's, at least one        of said ACM's comprising a gas-permeable membrane prepared by        the method of the first aspect of the invention and/or as        defined in the second and/or third aspect of the invention.        Generally, the package is such that at least 50%, often at least        75%, of the oxygen which enters the packaging atmosphere passes        through the one or more atmosphere control members.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawing, in which theFIGURE is a graph of R ratio (R) against volume fraction of particles(VFN) in Examples 1-4 below.

DETAILED DESCRIPTION OF THE INVENTION

In the Summary of the Invention above and in the Detailed Description ofthe Invention, the Examples, and the Statements below, reference is madeto particular features (including method steps) of the invention. It isto be understood that the disclosure of the invention in thisspecification includes all appropriate combinations of such particularfeatures. For example, where a particular feature is disclosed in thecontext of a particular aspect or embodiment of the invention, or aparticular Statement or claim, that feature can also be used, to theextent appropriate, in combination with and/or in the context of otherparticular aspects and embodiments of the invention, and in theinvention generally.

In describing and claiming the invention below, the followingabbreviations, definitions, and methods of measurement (in addition tothose already given) are used.

OTR is O₂ permeability. COTR is CO₂ permeability. OTR and COTR valuesare given in cc/100 inch²·atm·24 hrs, and can be measured using apermeability cell (supplied by Millipore) in which a mixture of O₂, CO₂and helium is applied to the sample, using a pressure of 0.035 kg/cm²(0.5 psi), and the gases passing through the sample are analyzed for O₂and CO₂ by a gas chromatograph. The cell could be placed in a water bathto control the temperature. The abbreviation P₁₀ is used to mean theratio of the permeability, to O₂ or CO₂ as specified, at a firsttemperature T₁° C. to the permeability at a second temperature T₂, whereT₂ is (T₁−10)° C. T₁ being 10° C. and T₂ being 0° C. unless otherwisenoted. The abbreviation R or R ratio is used to mean the ratio of COTRto OTR, both permeabilities being measured at 20° C. unless otherwisenoted. Pore sizes are measured by mercury porosimetry. Parts andpercentages are by weight, except for percentages of gases, which are byvolume. Temperatures are in degrees Centigrade. For crystallinepolymers, the abbreviation T_(o) is used to mean the onset of melting,the abbreviation T_(p) is used to mean the crystalline melting point,and the abbreviation ΔH is used to mean the heat of fusion. T_(o), T_(p)and ΔH are measured by means of a differential scanning calorimeter(DSC) at a rate of 10° C./minute and on the second heating cycle. T_(o)and T_(p) are measured in the conventional way well known to thoseskilled in the art. Thus T_(p) is the temperature at the peak of the DSCcurve, and T_(o) is the temperature at the intersection of the baselineof the DSC peak and the onset line, the onset line being defined as thetangent to the steepest part of the DSC curve below T_(p).

The term “comprises” and grammatical equivalents thereof are used hereinto mean that other elements (i.e. components, ingredients, steps etc.)are optionally present. For example, a composition “comprising” (or“which comprises”) ingredients A, B and C can contain only ingredientsA, B and C, or can contain not only ingredients A, B and C but also oneor more other ingredients. The term “consisting essentially of” andgrammatical equivalents thereof are used herein to mean that otherelements may be present which do not materially alter the claimedinvention. Where reference is made herein to a method comprising two ormore defined steps, the defined steps can be carried out in any order orsimultaneously (except where the context excludes that possibility), andthe method can include one or more other steps which are carried outbefore any of the defined steps, between two of the defined steps, orafter all the defined steps (except where the context excludes thatpossibility. The term “at least” followed by a number is used herein todenote the start of a range beginning with that number (which may be arange having an upper limit or no upper limit, depending on the variablebeing defined). For example “at least 1” means 1 or more than 1, and “atleast 80%” means 80% or more than 80%. The term “at most” followed by anumber is used herein to denote the end of a range ending with thatnumber (which may be a range having 1 or 0 as its lower limit, or arange having no lower limit, depending upon the variable being defined).For example, “at most 4” means 4 or less than 4, and “at most 40%” means40% or less than 40%. When, in this specification, a range is given as“(a first number) to (a second number)” or “(a first number)-(a secondnumber)”, this means a range whose lower limit is the first number andwhose upper limit is the second number. For example, “from 8 to 20carbon atoms” or “8-20 carbon atoms” means a range whose lower limit is8 carbon atoms, and whose upper limit is 20 carbon atoms. The numbersgiven herein should be construed with the latitude appropriate to theircontext and expression.

Where reference is made herein to sealed packages and sealed containers,and to sealing bags and other containers containing biologicalmaterials, it is to be understood that the sealing can be, but need notbe, hermetic sealing. Conventional methods for sealing bags and othercontainers can conveniently be used in this invention. If the bag issealed hermetically, it will generally be desirable to include one ormore pinholes in the bag, to achieve equilibration of the pressuresinside and outside the bag.

The method of the first aspect of the invention may optionally have oneor more of the following features:

-   -   (i) the hollow polymeric particles have one or more of the        following characteristics        -   (a) a maximum dimension D of most 0.5t, for example 0.1t to            0.4t, where t is the thickness of the solid coating;        -   (b) an average size of 0.2 to 0.8 micron, for example 0.4 to            0.7 micron; and        -   (c) at least 90% of the particles have a maximum dimension            of 0.2 to 0.8 micron, for example 0.4 to 0.7 micron;    -   (ii) the hollow particles dispersed in the liquid coating        composition are such that, at at least one temperature between 0        and 22° C., the gas-permeable membrane prepared by the method        has an R ratio which is at most 0.85 times, preferably at most        0.75 times, the R ratio of a gas-permeable membrane which is        produced by a method which is identical except that the liquid        coating composition does not contain the particles;    -   (iii) the hollow particles are hollow microspheres or hollow        microfilaments composed of a homopolymer or copolymer of        styrene, e.g. a copolymer of styrene and at least one acrylic        monomer;    -   (iv) the average size of the hollow particles dispersed in the        coating composition is 0.2 to 0.8 micron, for example 0.4 to 0.7        micron;    -   (v) at least 90% of the hollow particles dispersed in the        coating composition have a maximum dimension of 0.2 to 0.8        micron, for example 0.4 to 0.7 micron;    -   (vi) the coating composition contains 5 to 50%, preferably 10 to        40%, for example 20 to 35%, by weight of the hollow particles,        based on the combined weight of the polymer and the particles;    -   (vii) the volume of the hollow particles dispersed in the liquid        coating composition is        -   (a) at least 11%, preferably at least 12%, for example at            least 13%, of the volume of the solid coating, and/or        -   (b) less than 30%, for example less than 20%, of the volume            of the solid coating, and/or        -   (c) 11 to 20%, preferably 12 to 18%, of the volume of the            solid coating;    -   (viii) the polymer comprises a crystalline polymer        -   (a) having a peak melting temperature T_(p) of −5 to 40° C.,            for example 0 to 25° C., and a heat of fusion of at least 5            J/g, preferably least 10 J/g, especially at least 20 J/g,            and/or        -   (b) having an onset of melting temperature T_(o) such that            (T_(p)−T_(o)) is less than 10° C., preferably less than 8°            C., for example 5-10° C., and/or        -   (c) comprising at least one side chain crystalline (SCC)            polymer, for example an SCC polymer which contains            ethylenically unsaturated repeating units;    -   (ix) the polymer is an amorphous polymer, e.g. a polysiloxane;    -   (x) the polymer becomes crosslinked during the step (B);    -   (xi) the coating composition comprises a liquid carrier, for        example an aqueous liquid (including water) having the polymer        and the hollow particles uniformly dispersed therein, preferably        a mixture of an aqueous emulsion of the polymer and an aqueous        emulsion of the hollow particles; and    -   (xii) step (B) comprises heating the coating, for example to        remove a liquid carrier therefrom and/or to crosslink the        polymeric matrix and/or to cause the hollow particles to fuse to        each other and/or to the polymeric matrix; the heating can be        carried out as a separate step or as part of a continuous        operation; the coating can for example be heated at a        temperature of 50 to 85° C.

Membranes prepared by the method of the first aspect of the inventionmay optionally have one or more of the following characteristics

-   -   (a) an OTR at 20° C. of at least 30,000, preferably at least        50,000, cc/100 in²·atm·24 hrs;    -   (b) an oxygen P₁₀ ratio of at least 2, preferably at least 2.5,        over at least one 10° C. temperature range between 0 and 25° C.;    -   (c) a carbon dioxide P₁₀ ratio of at least 2, preferably at        least 2.5, over at least one 10° C. temperature range between 0        and 25° C.; and    -   (d) an R ratio of less than 4, preferably less than 3,        particular less than 2.5, at at least one temperature between 0        and 22° C.        If higher OTR and COTR values are desired, the coating weight of        the coating composition can be reduced, but this will result in        lower P₁₀ values.

The gas-permeable membranes of the second and third aspects of theinvention may optionally have one or more of the followingcharacteristics:

-   -   (i) the solid polymeric coating comprises microscopic voids        and/or hollow polymeric particles such that, at at least one        temperature between 0 and 22° C., the membrane has an R ratio        which is at most 0.85 times, preferably at most 0.75 times, the        R ratio of a membrane which is the same except that the coating        does not contain the microscopic voids and/or hollow polymeric        particles;    -   (ii) the hollow polymeric particles have one or more of the        following characteristics        -   (a) a maximum dimension D of most 0.5t, for example 0.1t to            0.4t, where t is the thickness of the solid coating;        -   (b) an average size of 0.2 to 0.8 micron, for example 0.4 to            0.7 micron; and        -   (c) at least 90% of the particles have a maximum dimension            of 0.2 to 0.8;    -   (iii) the solid coating contains 5 to 50%, preferably 10 to 40%,        for example 20 to 35%, by weight of the second polymer;    -   (iv) the hollow polymeric particles and/or the microscopic voids        resulting from fusion of hollow polymeric particles define        volumes which constitute        -   (a) at least 11%, preferably at least 12%, for example at            least 13%, of the volume of the solid coating, and/or        -   (b) less than 30%, for example less than 20%, of the volume            of the solid coating, and/or        -   (c) 11 to 20%, preferably 12 to 18%, of the volume of the            solid coating;    -   (v) the polymeric matrix comprises a crystalline polymer as        defined in subparagraph (vii) above;    -   (vi) the polymeric matrix is crosslinked;    -   (vii) the membrane has at least one of the following        characteristics        -   (a) an OTR at 20° C. of at least 30,000, preferably at least            50,000, cc/100 in²·atm·24 hrs;        -   (b) an oxygen P₁₀ ratio of at least 2, preferably at least            2.5, over at least one 10° C. temperature range between 0            and 25° C.;        -   (c) a carbon dioxide P₁₀ ratio of at least 2, preferably at            least 2.5, over at least one 10° C. temperature range            between 0 and 25° C.; and        -   (d) an R ratio of less than 4, preferably less than 3,            particular less than 2.5, at at least one temperature            between 0 and 22° C.

The microporous polymeric film, which serves as a support for thepolymeric coating, comprises a network of interconnected pores such thatgases can pass through the film. Preferably the pores have an averagepore size of less than 0.24 micron. Other optional features of themicroporous film include

-   -   (a) at least 70%, e.g. at least 90%, of the pores having a pore        size of less than 0.24 micron;    -   (b) at least 80% of the pores have a pore size less than 0.15        micron;    -   (c) less than 20% of the pores have a pore size less than 0.014        micron;    -   (d) the pores constitute 35 to 80% by volume of the microporous        film;    -   (e) the microporous film comprises a polymeric matrix        comprising (i) an essentially linear ultrahigh molecular weight        polyethylene having an intrinsic viscosity of at least 18        deciliters/g, or (ii) an essentially linear ultrahigh molecular        weight polypropylene having an intrinsic viscosity of at least 6        deciliters/g, or (iii) a mixture of (i) and (ii);    -   (f) the microporous film contains 30 to 90% by weight, based on        the weight of the film, of a finely divided particulate        substantially insoluble filler, preferably a siliceous filler,        which is distributed throughout the film;    -   (g) the microporous film is prepared by a process comprising        -   (A) preparing a uniform mixture comprising the polymeric            matrix material in the form of a powder, the filler, and a            processing oil;        -   (B) extruding the mixture as a continuous sheet;        -   (C) forwarding the continuous sheet, without drawing, to a            pair of heated calender rolls;        -   (D) passing the continuous sheet through the calender rolls            to form a sheet of lesser thickness;        -   (E) passing the sheet from step (D) to a first extraction            zone in which the processing oil is substantially removed by            extraction with an organic extraction liquid which is a good            solvent for the processing oil, a poor solvent for the            polymeric matrix material, and more volatile than the            processing oil;

2(F) passing the sheet from step (E) to a second extraction zone inwhich the organic extraction liquid is substantially removed by steam orwater or both; and

-   -   -   (G) passing the sheet from step (F) through a forced air            dryer to remove a residual water and organic extraction            liquid.

As indicated above, the polymeric matrix of the coating on themicroporous film preferably comprises, and may consist essentially of, acrystalline polymer, preferably an SCC polymer. The use of a crystallinepolymer results in an increase in the P₁₀ values in the melting regionof the polymer. The SCC polymer can comprise, and optionally can consistof, units derived from (i) at least one n-alkyl acrylate or methacrylate(or equivalent monomer, for example an amide) in which the n-alkyl groupcontains at least 12 carbon atoms, for example in amount 35-100%,preferably 50-100%, often 80-100%, and optionally (ii) one or morecomonomers selected from acrylic acid, methacrylic acid, and esters ofacrylic or methacrylic acid in which the esterifying group contains lessthan 10 carbon atoms. The SCC polymer can also include units derivedfrom a diacrylate or other crosslinking monomer. The preferred number ofcarbon atoms in the alkyl group of the units derived from (i) dependsupon the desired melting point of the polymer. For the packaging ofbiological materials, it is often preferred to use a polymer having arelatively low melting point, for example a polymer in which the alkylgroups in the units derived from (i) contain 12 and/or 14 carbon atoms.The SCC polymer can be a block copolymer in which one of blocks is acrystalline polymer as defined and the other block(s) is crystalline oramorphous, for example a block copolymer comprising (i) polysiloxanepolymeric blocks, and (ii) crystalline polymeric blocks having a T_(p)of −5 to 40° C. Preferred SCC polymers are those prepared by emulsionpolymerization, particularly those prepared in accordance with thedisclosure of U.S. Pat. Nos. 6,199,318 (Stewart et al) and 6,540,984(Stewart et al.), the entire disclosures of which are incorporatedherein by reference.

The polymeric matrix can also consist of or contain other crystallineand amorphous polymers. Examples of such other polymers includecis-polybutadiene, poly (4-methylpentene), polysiloxanes includingpolydimethyl siloxane, and ethylene-propylene rubber.

The preferred hollow polymeric particles for use in this invention arehollow microspheres of an organic polymer. Such microspheres can beconsist essentially of, for example, homopolymers of styrene; copolymersof styrene and one or more other monomers, for example styrene acryliccopolymers, styrene divinylbenzene copolymers, styrene maleic anhydridecopolymers, and styrene butadiene copolymers; polyvinyl toluene; andpolymethyl methacrylate. Such particles are commercially available in awide range of sizes as opacifiers for paints and for use in cytometry.For example, acrylic/styrene copolymers are available under thetradename Ropaque from Rohm & Haas; polystyrene and carboxylmicrospheres are available under the tradename Polybead fromPolysciences Inc.; and polystyrene and styrene copolymer microspheresare available from Bangs Laboratories Inc. For use in this invention,the particles of preferably in the form of an aqueous emulsion thatblends easily with an aqueous emulsion of the matrix polymer to becoated onto the microporous film.

The permeability of the containers and packages of the invention can beinfluenced by perforating the container in order to make a plurality ofpinholes therein.

EXAMPLES

The invention is illustrated in the following Examples, Examples C1-C4being comparative Examples. In the Examples, the SCC1, SCC2 and SCC3acrylate polymers used to provide the polymeric matrix in the coatingswere prepared by emulsion polymerizing the monomers and parts by weightthereof shown in Table 1 to give emulsion polymers having the % solids,particle sizes, T_(p) and ΔH also shown in Table 1. In Table 1, MAA ismethacrylic acid, C6DA is hexyldiacrylate, C12A is dodecyl acrylate, andC14A is tetradecyl acrylate.

TABLE 1 % particle T_(p) ΔH MAA C6DA C12A C14A solids size (nm) ° C. J/gSCC1 3.96 1.0 11.5 83.6 30.5 110 16.85 SCC2 4.0 0.7 38.2 57.2 48.1 16510.16 35.8 SCC3 4.0 0.7 0 95.4 47.2 123 19.6 44.8OP96 is an aqueous emulsion containing about 36.6% or about 47.2% byweight of hollow polymer spheres having an average particle size of 550nm. It is available from Rohm & Haas under the tradename Ropaque OP96.Teslin is a microporous polyethylene film available commercially fromPPG under the tradename Teslin SP7. It contains about 60% silica, has athickness of about 0.18 mm (0.007 inch), a porosity of about 65%, anaverage pore size of about 0.1 micron and a largest pore size of 4-10microns. The distribution of pore sizes in Teslin SP7 is set out inTable 2 below.

TABLE 2 Pore Size (microns) .013 .016 .026 .044 .058 .08 .11 .15 .24 .36.6 % of pores larger 90 80 70 60 50 40 30 20 10 5 2 than pore size

In each of the Examples, the coating composition was coated onto Teslinusing a #10 wire-wound rod, and was then dried at 82° C. for 2 hours,resulting in a crosslinked coating on the surface of the Teslin. The OTRand COTR of the resulting product were measured at differenttemperatures.

Examples C1 and 1-4

Examples C1 and 1-4 are summarized in Table 3 below. In each of theseExamples, the coating composition (cc) was prepared by mixing polymerSCC1 and the indicated percentage by weight (based on the weight of themixture) of 0P96 (36.6% solids), followed by dilution to about 3% solidsin Examples 1-4 and to about 7% solids in Example C1. The dried coating(dc) containing the indicated percentages by weight and by volume of thehollow polymeric spheres.

TABLE 3 Wt % Wt % vol % Temperature Ex OP96 spheres spheres 22° C. 10°C. 0° C. P₁₀ # in cc in dc in dc 0TR R 0TR R 0TR R (O₂) C1 0 0 0 45.15.21 12.2 4.99 6.4 2.21 1.90 1 20 23.1 10.8 55.8 5.16 16.9 4.06 9.7 2.031.74 2 23 26.4 12.7 42.8 4.94 16.0 4.20 9.8 2.86 1.64 3 25 28.6 13.951.0 4.27 26.1 3.14 20.7 4.09 1.32 4 30 34 17.2 48.0 4.30 23.7 3.36 18.03.94 1.28

Examples C2 and 5

Examples C2 and 5 are summarized in Table 4 below. In each of theseExamples, the coating composition (cc) was prepared by mixing polymerSCC2 and the indicated percentage by weight (based on weight of themixture) of 0P96 (55% solids), followed by dilution to about 5% solidsin Example C2 and about 12% solids in Example 5. The dried coating (dc)containing the indicated percentages by weight and by volume of hollowpolymeric particles.

TABLE 4 P₁₀ P₁₀ wt % wt % vol % Temperature O₂ CO₂ Ex OP96 spheresspheres 22° C. 10° C. 0° C. 0-10 0-10 # in cc in dc in dc 0TR R 0TR R0TR R ° C. ° C. C2 0 0 0 71 4.96 46.3 5.41 14.0 3.86 3.31 4.69 5 24 19.413.3 85.1 3.88 61.9 3.84 34.5 2.18 1.81 3.17

Examples C3, C4 and 6

Examples C3, C4 and 6 are summarized in Table 5 below. In Example C3,the coating composition (cc) contained only polymer SCC3 and was coatedat 11% solids. In Example C4, the coating composition contained a 50/50mixture of polymers SCC2 and SCC3, and was coated at 10% solids. InExample 6, the coating composition was made by mixing 76% of a 50/50mixture of polymers SCC2 and SCC3, and 24% of OP96 (36.6%) followed bydilution to about 12% solids. The dried coating (dc) contain theindicated percentages by weight and by volume of hollow polymericspheres.

TABLE 5 Wt % wt % vol % Temperature Ex OP96 spheres spheres 25° C. 20°C. 15° C. 10° C. # in cc in dc in dc 0TR R 0TR R 0TR R 0TR R C3 0 0 079.3 4.64 70.8 4.79 31.3 4.49 19.3 2.86 C4 0 0 0 125 3.82 111 3.86 80.43.66 58.1 3.03 6 24 19.5 13.3 107 3.22 101 3.19 75.7 2.79 62.4 2.58 Wt %Wt % vol % Temperature Ex OP96 spheres spheres 5° C. 0° C. # in cc in dcin dc 0TR R 0TR R C3 0 0 0 16.7 2.34 14.4 1.88 C4 0 0 0 42.3 2.2 38.41.61 6 24 19.5 13.3 52.7 1.76 47.6 1.39 P₁₀ (O₂) P₁₀ (CO₂) Wt % wt % vol% 15- 10- 15- 10- Ex OP96 spheres spheres 25 20 5-15 0-10 25 20 5-150-10 # in cc in dc in dc ° C. ° C. ° C. ° C. ° C. ° C. ° C. ° C. C3 0 00 2.58 3.71 1.89 1.36 2.66 6.14 3.6 2.05 C4 0 0 0 1.55 1.92 1.78 1.512.62 2.45 2.95 2.85 6 24 19.5 13.3 1.41 1.61 1.44 1.31 1.63 2.00 2.282.44

Examples C5, C6, 7 and 8

In Examples C5, C6, 7 and 8, packages containing 3 lb (2.25 kg) of wholestrawberries were used. In Example C5, the package was open. In ExampleC6, the package was sealed and was composed of Mylar except for a knownatmosphere control member having an area of 2.5 inch² (1610 mm²) andcomposed of Teslin having a coating thereon of an SCC polymer containingunits derived from tetradecyl acrylate (57 parts), hexadecyl acrylate(40 parts) and acrylic acid (3 parts). In Example 7, the package was thesame as in Example C6, except that the atmosphere control member wascomposed of the coated film of Example 5. In Example 8, the package wasthe same as in Example 7, except that the area of the atmosphere controlmember was 4.0 inch² (2580 mm²). Each package in Examples C6, 7 and 8had a 26 g pinhole to equalize the pressures inside and outside thesealed package.

The packages were stored at 2° C. for 72 hours, then at 10° C. for 48hours, and finally at 2° C. for 120 hours, after which the sealedpackages were opened. Table 6 below shows the weight loss of thestrawberries at the end of the storage period and the O₂ and CO₂contents of the atmosphere within the sealed packages at 10° C. after120 hours (i.e. at the end of the 10° C. storage period) and at 2° C.after 144 hours.

TABLE 6 Ex at 10° C. after 120 hours at 2° C. after 144 hours wt lossafter # O₂ CO₂ O₂ CO₂ 240 hours C5 — — — — 21.8 C6 3.9 5.5 7.3 3.4 0.3 75.0 8.9 7.5 8.4 0.16 8 8.9 6.4 11.2 6.1 0.21

1. A gas-permeable membrane which comprises (1) a microporous film, and(2) a solid coating on the microporous film, the coating comprising (a)a matrix comprising a first polymer, and (b) hollow particles which (i)are composed of a polymeric composition comprising a second polymerwhich is different from the first polymer, (ii) are dispersed in thematrix, and (iii) have a maximum dimension which is at most 50% of thethickness of the solid coating.
 2. A membrane according to claim 1wherein the solid coating also comprises a plurality of microscopicvoids which (i) provide continuous pathways for the transmission ofoxygen and carbon dioxide through the coating, and (ii) are at leastpartly defined by walls composed of the second polymer.
 3. A membraneaccording to claim 1 wherein (i) the first polymer consists essentiallyof at least one side chain crystalline polymer having a peak meltingtemperature T_(p) of −5 to 40° C., and a heat of fusion of at least 10J/g, and (ii) the second polymer consists essentially of a homopolymeror copolymer of styrene.
 4. A membrane according to claim 1 which (a)has an oxygen permeability (OTR) at 20° C. of at least 30,000 cc/100in²·atm·24 hrs; (b) has an oxygen P₁₀ ratio of at least 2 over at leastone 10° C. temperature range between 0 and 25° C.; (c) has a carbondioxide P₁₀ ratio of at least 2 over at least one 10° C. temperaturerange between 0 and 25° C.; and (d) has at at least one temperaturebetween 0 and 22° C. an R ratio less than
 4. 5. A membrane according toclaim 1 wherein the solid coating contains 5 to 50% by weight of thesecond polymer.
 6. A membrane according to claim 1 wherein the solidcoating contains 10 to 40% by weight of the second polymer.
 7. Amembrane according to claim 1 wherein at least 90% of the particles havea maximum dimension of 0.2 to 0.8 micron.
 8. A membrane according toclaim 1 wherein the volume of the hollow particles dispersed in thesolid coating is 11 to 20% of the volume of the solid coating.
 9. Agas-permeable membrane which comprises (1) a microporous film, and (2) asolid coating on the microporous film, the coating comprising (a) amatrix comprising a first polymer which is at least one side chaincrystalline polymer having a peak melting temperature T_(p) of −5 to 40°C., and a heat of fusion of at least 10 J/g, and (b) hollow particleswhich (i) are composed of a polymeric composition comprising a secondpolymer which is different from the first polymer, (ii) are dispersed inthe matrix, and (iii) have a maximum dimension which is at most 50% ofthe thickness of the solid coating.
 10. A membrane according to claim 9wherein the solid coating also comprises a plurality of microscopicvoids which (i) provide continuous pathways for the transmission ofoxygen and carbon dioxide through the coating, and (ii) are at leastpartly defined by walls composed of the second polymer.
 11. A membraneaccording to claim 1 wherein the second polymer consists essentially ofa homopolymer or copolymer of styrene.
 12. A membrane according to claim9 which (a) has an oxygen permeability (OTR) at 20° C. of at least30,000 cc/100 in²·atm·24 hrs; (b) has an oxygen P₁₀ ratio of at least 2over at least one 10° C. temperature range between 0 and 25° C.; (c) hasa carbon dioxide P₁₀ ratio of at least 2 over at least one 10° C.temperature range between 0 and 25° C.; and (d) has at at least onetemperature between 0 and 22° C. an R ratio less than
 4. 13. A membraneaccording to claim 9 wherein the solid coating contains 5 to 50% byweight of the second polymer.
 14. A membrane according to claim 9wherein the solid coating contains 10 to 40% by weight of the secondpolymer.
 15. A membrane according to claim 9 wherein at least 90% of theparticles have a maximum dimension of 0.2 to 0.8 micron.
 16. A membraneaccording to claim 9 wherein the volume of the hollow particlesdispersed in the solid coating is 11 to 20% of the volume of the solidcoating.
 17. A method of preparing a gas-permeable membrane whichcomprises a microporous film and a solid polymeric coating on themicroporous film, the method comprising (A) forming a liquid coating onthe microporous film, the liquid coating being composed of liquidcoating composition which comprises (a) a first polymer, and (b) hollowparticles which (i) are dispersed in the coating composition, and (ii)are composed of a polymeric composition comprising a second polymer, thesecond polymer being different from the first polymer; and (B)solidifying the liquid coating on the microporous film; the hollowpolymeric particles having a maximum dimension which is at most 50% ofthe thickness of the solid coating.
 18. A method according to claim 19wherein step (B) comprises heating which at least partially melts atleast some of the hollow polymeric particles so that they fuse togetherto form a plurality of microscopic voids.
 19. A method according toclaim 18 which comprises heating the coating at a temperature of 50 to85° C.
 20. A container which can be sealed around a respiring biologicalmaterial or has been sealed around a respiring biological material, andwhich includes one or more atmosphere control members, at least one ofthe atmosphere control members comprising a gas-permeable membraneselected from gas-permeable membranes as defined in claim 1,gas-permeable membranes as defined in claim 9, and gas-permeablemembranes prepared by the method of claim 17.